The present invention relates to a method for on-wafer testing of microwave devices and, in particular, to a method for testing a photodiode that is connected, at one end, to a probeable contact on the upper side of the wafer and, at the other end, to a ground plane on the lower side of the wafer.
When testing microwave devices, there is often a choice made either to test each device "on-wafer" or, in the alternative, to dice the wafer and to separately package each device for measurement. This latter approach is time-consuming, however, due to the extra interconnecting and packaging steps involved. Moreover, the interconnect structures that are used, such as bond wires, add their own electrical effects which can only be compensated for at additional expense. Furthermore, if measurements are made that indicate inferior performance by particular devices, only the "on-wafer" approach permits further processing to occur to improve such performance. For these reasons, on-wafer testing, where feasible, has been the preferred testing approach for high speed wafer devices.
The feasibility of on-wafer testing of a three-terminal transistor by a pair of probes was described by Arthur Fraser, Reed Gleason and Eric Strid in "GHz ON-SILICON WAFER PROBING CALIBRATION METHODS," Proceedings of the 1988 Bipolar Circuits and Technology Meeting, Paper 7.6, Pub. No. 88CH2592-4, pp. 154-157, Jul. 1988. The described set-up included setting the ground conductors of one probe at a lower potential than the corresponding ground conductors of the other probe so that a pair of adjacent transistors, one on each side of the transistor under test, could effectively shunt the ground conductors of each probe together through their forward biased base-emitter junctions. The remaining collector terminal of each transistor was returned to the ground plane on the underside of the wafer. This two-probe measurement approach, however, has not appeared of particular relevance to the testing of two-terminal devices, such as photodiodes, which are accessible only by a single electrical probe at a time.
A method for on-wafer testing of photodiodes has been described by John Modolo, Gordon Anderson, Francis Kub, and Ingham Mack in "Wafer Level High-Frequency Measurements of Photodetector Characteristics," Applied Optics, Vol. 27, pp. 3059-3061, Aug. 1988. In this method, the signal and ground conductors of the probe were placed directly onto the pair of upper contacts between which the photodiode was connected, and an optical probe was used to apply light to the photodiode. The particular approach used to calibrate the measurements made in this setup was not, however, described in any detail, although a standard lightwave analyzer calibration set appeared to be involved. Such sets have customarily included, for reference purposes, a photoreceiver of known characteristic which is attachable in place of the microwave probe at the coaxial termination of the interconnecting cable. To calibrate for the effects of the probe in the described setup, however, reference is needed to the probe tip and requires, for example, the use of a probeable photodiode standard, an additional accessory that would need to be specially purchased for making this type of measurement.
A further difficulty with the above-described method is its lack of adaptability to other commonly encountered device configurations, such as where the photodiode, rather than having both ends terminated on the upper side of the wafer, instead has one end terminated to the ground plane on the lower side of the wafer. In this configuration, the noncoplanar ends of the photodiode are not directly accessible by a standard microwave probe having coplanar probe leads, thereby requiring the use of a specially constructed probe.
Accordingly, it is an object of the present invention to provide a method for accurate testing of two-terminal microwave devices on-wafer and without the use of special equipment or accessories.
A related object of the present invention is to permit the use of a standard microwave probe, having coplanar conductors at its probe tip, for on-wafer testing of two-terminal microwave devices, particularly those of the type that connect, at one end, to a single probeable contact on the upper side of the wafer and connect, at their other end, with a common ground plane on the lower side of the wafer.
Another related object is to enable accurate testing using a standard microwave probe where the device under test is a photodetector and an optical characteristic is being measured.
A further object of the present invention is to facilitate calibration of the on-wafer measurement of an optical characteristic of a photodetector without the use of a probeable photodetector standard.
Another object of the present invention is to facilitate calibration of the on-wafer measurement of a two-terminal microwave device to correct for errors introduced by taking the measurement through a nonideal ground path.